Inquiring Minds

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3.  Lots about Virus from Two Web Sites

All about Cookies

The Web. "State" in this case refers to an application's ability to work interactively with a user, remembering all data since the application started, and differentiating between users and their individual data sets.

An analogy I like to use is a laundry cleaner's shop. You drop something off, and get a ticket. When you return with the ticket, you get your clothes back. If you don't have the ticket, then the laundry man doesn't know which clothes are yours. In fact, he won't be able to tell whether you are there to pick up clothes, or a brand new customer. As such, the ticket is critical to maintaining state between you and the laundry man.

Unfortunately, HTTP is a "stateless" protocol. This means that each visit to a site (or even clicks within a site) is seen by the server as the first visit by the user. In essence, the server "forgets" everything after each request, unless it can somehow mark a visitor (that is, hand him a "laundry ticket") to help it remember. Cookies can accomplish this.

1.1 What is a Cookie?

A cookie is a text-only string that gets entered into the memory of your browser. This value of a variable that a website sets. If the lifetime of this value is set to be longer than the time you spend at that site, then this string is saved to file for future reference.

1.2 Where did the term cookies come from?

According to an article written by Paul Bonner for Builder.Com on 11/18/1997:

"Lou Montulli, currently the protocols manager in Netscape's client product division, wrote the cookies specification for Navigator 1.0, the first browser to use the technology. Montulli says there's nothing particularly amusing about the origin of the name: 'A cookie is a well-known computer science term that is used when describing an opaque piece of data held by an intermediary. The term fits the usage precisely; it's just not a well-known term outside of computer science circles.'"

http://www.cookiecentral.com/faq/#1.2 contains the following subjects:

• 1. INTRODUCTION
• 1.1 What is a cookie?
• 1.2 Where did the term cookies come from?
• 1.3 Why do sites use cookies?
• 1.4 Where can I get more information?
• 2. GENERAL QUESTIONS/MISCELLANEOUS
• 2.1 Introduction
• 2.2 Can I delete cookies?
• 2.3 How do I set my browser to reject cookies?
• 2.4 Are cookies dangerous to my computer?
• 2.5 Will cookies fill up my hard drive?
• 2.6 Are cookies a threat to my privacy?
• 2.7 Sites are telling me I need to turn on cookies, but they are on. What's wrong?
• 2.8 I deleted my cookies, and I can't log-on to my favorite site anymore. What can I do?
• 2.9 How did I get a cookie from doubleclick? I've never been there!
• 2.10 I looked at my Internet Explorer cookies, and they had my username on them! Can servers see my username?
• 2.11 There are two extra files in my Cookies folder called Mm256.dat and Mm2048.dat. What are they?
• 3. COOKIE FUNDAMENTALS
• 3.1 Introduction
• 3.2 How does a cookie really work?
• 3.3 Breakdown of Cookie Parameters
• 3.4 How do cookies end up on my hard drive?
• 3.5 What are all those entries in my cookies.txt file?
• 3.6 Where does MSIE keep its cookies?
• 3.7 Are cookies Year 2000 Compliant?
• 4. ADVANCED TOPICS
• 4.1 Introduction
• 4.2 Creating a cookie value
• 4.3 Retrieving a cookie value
• 4.4 Clearing a cookie value
• 4.5 Detecting if cookies are accepted
• 4.6 Compact Privacy Policies and IE6

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Every machine on the Internet has a unique identifying number, called an IP Address. A typical IP address looks like this:

• 216.27.61.137

To make it easier for us humans to remember, IP addresses are normally expressed in decimal format as a "dotted decimal number" like the one above. But computers communicate in binary form. Look at the same IP address in binary:

• 11011000.00011011.00111101.10001001

The four numbers in an IP address are called octets, because they each have eight positions when viewed in binary form. If you add all the positions together, you get 32, which is why IP addresses are considered 32-bit numbers. Since each of the eight positions can have two different states (1 or 0) the total number of possible combinations per octet is 2⁸ or 256. So each octet can contain any value between 0 and 255. Combine the four octets and you get 2³² or a possible 4,294,967,296 unique values!

Out of the almost 4.3 billion possible combinations, certain values are restricted from use as typical IP addresses. For example, the IP address 0.0.0.0 is reserved for the default network and the address 255.255.255.255 is used for broadcasts.

The octets serve a purpose other than simply separating the numbers. They are used to create classes of IP addresses that can be assigned to a particular business, government or other entity based on size and need. The octets are split into two sections: Net and Host. The Net section always contains the first octet. It is used to identify the network that a computer belongs to. Host (sometimes referred to as Node) identifies the actual computer on the network. The Host section always contains the last octet. There are five IP classes plus certain special addresses:

• Default Network - The IP address of 0.0.0.0 is used for the default network.
• Class A - This class is for very large networks, such as a major international company might have. IP addresses with a first octet from 1 to 126 are part of this class. The other three octets are used to identify each host. This means that there are 126 Class A networks each with 16,777,214 (2²⁴ -2) possible hosts for a total of 2,147,483,648 (2³¹) unique IP addresses. Class A networks account for half of the total available IP addresses. In Class A networks, the high order bit value (the very first binary number) in the first octet is always 0.

• Loopback - The IP address 127.0.0.1 is used as the loopback address. This means that it is used by the host computer to send a message back to itself. It is commonly used for troubleshooting and network testing.
• Class B - Class B is used for medium-sized networks. A good example is a large college campus. IP addresses with a first octet from 128 to 191 are part of this class. Class B addresses also include the second octet as part of the Net identifier. The other two octets are used to identify each host. This means that there are 16,384 (2¹⁴) Class B networks each with 65,534 (2¹⁶ -2) possible hosts for a total of 1,073,741,824 (2³⁰) unique IP addresses. Class B networks make up a quarter of the total available IP addresses. Class B networks have a first bit value of 1 and a second bit value of 0 in the first octet.

• Class C - Class C addresses are commonly used for small to mid-size businesses. IP addresses with a first octet from 192 to 223 are part of this class. Class C addresses also include the second and third octets as part of the Net identifier. The last octet is used to identify each host. This means that there are 2,097,152 (2²¹) Class C networks each with 254 (2⁸ -2) possible hosts for a total of 536,870,912 (2²⁹) unique IP addresses. Class C networks make up an eighth of the total available IP addresses. Class C networks have a first bit value of 1, second bit value of 1 and a third bit value of 0 in the first octet.

• Class D - Used for multicasts, Class D is slightly different from the first three classes. It has a first bit value of 1, second bit value of 1, third bit value of 1 and fourth bit value of 0. The other 28 bits are used to identify the group of computers the multicast message is intended for. Class D accounts for 1/16^th (268,435,456 or 2²⁸) of the available IP addresses.

• Class E - Class E is used for experimental purposes only. Like Class D, it is different from the first three classes. It has a first bit value of 1, second bit value of 1, third bit value of 1 and fourth bit value of 1. The other 28 bits are used to identify the group of computers the multicast message is intended for. Class E accounts for 1/16^th (268,435,456 or 2²⁸) of the available IP addresses.